Rotary drilling apparatus is employed in the petroleum industry to drill holes in which dynamite explosives are set off for seismic exploration.
Portable drilling rigs have heretofore been developed for use in remote locations where it is difficult to transport and locate more conventional earth drilling equipment. A recently developed portable drilling rig that is helicopter-transported is disclosed in Womack et al. U.S. Pat. No. 4,192,393. The rig disclosed in the Womack et al. patent features three modular components each weighing less than 1300 pounds so as to be capable of transportation by a relatively inexpensive helicopter. Prior known derricks used in portable drilling rigs have required a derrick with chains and sprockets or continuous screw structure to move the load. The disadvantage of this structure is that it is relatively heavy, readily becomes dirty so as to require cleaning, and further is relatively slow in operation.